Automatic gain control circuit



July 16, 1940. A. ANDREATTA AUTOMATIC GAIN CON TROL CIRCUIT Filed May 14, I938 l INVENTOR WIT/l co mm 4 m w/mR A m M 1? 0 A F 7 m Patented July 16, 1940 2 208 1 outrun s'rares PATENT OFFICE AUToMATIC GAIN CONTROL CIRCUIT Alfons Andreatta, Berlin, Germany, assignor to a General Electric Company, a corporation of a New York Application May 14, 1938, Serial No. 207,898 In Germany June 25, 1937 3 Claims. (01. 250- 20) The present circuit organization serves to inpotential, regardless of what value the incident sure automatic stabilization of the audio output radio potential may be. In other words, if the potential, or power, in a radio frequency appadifferential potential preserves its uniform and ratus, for instance, of a receiver set, in the presstable time-shape, t s fa W nsu a p 5 ence of fluctuations of the radio potential acting fectly automatic voltage regulation. At the grid 5 at the input of the apparatus. It of tube H arises a constant potential, and in The present scheme diners from circuits comthe plate C t O is tube, therefore, a prising exponential tubes insofar as the fluctuastant output power, regardless of the size of the tions of the output potential can be easily kept d t Signal energy.

inside narrow limits, say, :20 per cent, whereas The lack of symmetry of the circuits which is 10 in simple circuit arrangements including reguqu ed for the Stab li a on O the e u a lating tubes the resulting fluctuations oi the outvoltage may be realized in various ways. For input potential are far greater, say at the ratio of stance, one diode may be subjected to a biasing 1:3 or even 1:5. The novel circuit organization p te i l. Another plan Would e to d p One here disclosed further offers the merit that a of the diode tuning circuits. Also diodes pre- 15 wide range of control is realizable. Inasmuch as senting dissimilar slopes of their characteristic for control action itself the audio alternating could be used, or the load resistances 8 and 9 voltage resulting from rectification is used, the could be made of dissimilar size. Finally, the circuit scheme is useful only if the radio potential clos ess f oup With t e o l 3 0f the two i m d lat d, circuits 4 and 5 could be made dissimilar. As a 20 In the drawing: general rule, two or more of the said steps are Fig. 1 shows a circuit embodying the invention, combined more preferably in order that the opti- Fig. 2 shows curves illustrating the action of the mum regulator curve form may be secured. control circuit, In the exemplified embodiment here illustrated,

Referring to the drawing, I denotes the signal the two circuits iand 5 are coupled to coil 3 25 7 input circuit, 2 is a radio amplifier tube. The equally closely. A diiierence in the radio potenplate circuit of this tube comprises a coil 3 to tials is obtained in the two circuits by connecting which two tuned circuits 4 and 5 are coupled. a damping resistance l2 in parallel relation to the The radio energy is rectified by the two diodes 6 coil 5. Moreover, the diode I is impressed from 0- and I. Now, arrangements could be made in potentiometer 13 with a direct current voltage so such a way that the diode 6 cuts off, for instance, as to bias the same. The effect of this biasing the bottom alternations of the radio current by Voltage is that the regulator curve as shown in rectification. Incidentally, the coupling of circuit Fig. 2 of the drawing initially rises rather rapidly 5 and the polarity of the diode I may be so chosen and steeply, and as a result the desired constancy 5 that rectification by diode i will cut off the upper of the output Voltage is attained at a fast rate. alternations of the radio waves. The modula- Fig. 2 shows the regulator curve as obtained by tions of the two diodes, as will thus be seen, are measurement in a circuit organization of the kind of like value, but displaced 180 degrees in phase. shown in Fig. 1. If care is taken so that no phase If the two potentials arising across the resistdistortions arise in the novel circuit organization 7 ances 8 and 9 are impressed upon a common rehere disclosed, the same scheme will be found 0 sistance, say the grid leak 10' of tube ll, there useful for automatic volume control in broadcast results an audio potential whose sum total is receiver sets. zero. However, zero voltage results only in the What is claimed is: presence of perfect symmetry of the circuit or- 1. In combination with a modulated carrier ganization. What is of interest here is lack of amplifier and a modulation amplifier, a demodu- 45 symmetry for which the constituent potentials lation network consisting of two rectifiers each are of dissimilar size. In such a case the sum having a carrier input circuit coupled to the car- Voltage is of a value other than zero, and the rier amplifier output, said rectifiers being concurve-shape of the same will correspond exactly nected in opposition, means for impressing the to the modulation voltage at the grid of tube 2. differential modulation voltage output of said 50 The higher the radio potential at the tube 2, the rectifiers upon the input of said modulation higher will be the voltages across the resistances amplifier, d rectifiers being n ructed and a and e. If, then, care be taken so that the differrran ed to provide a differential ta whose ence of these voltages stays constant, there will V e increases With Carrie! p ude ncrease always results the same curve for the differential at such a rate as to maintain the modulation 55 amplifier output substantially constant, each of said rectifiers being of the diode type, and one of the diodes having a delay bias applied thereto and a damping resistor being connected in'the carrier input circuit of said delayed diode.

2. In combination with a modulated carrier amplifier and a modulation amplifier, a demodulation network consisting of two rectifiers each having a carrier input circuit coupled to the carrier amplifier output, said rectifiers being connected in opposition, means for impressing the differential modulation voltage output of said rectifiers upon the input of said modulation amplifier,

said .rectifiers being constructed and arranged to provide a differential voltage whose value increases with carrier amplitude increase at such a rate as to maintain the modulation amplifier output substantially constant, and one of said rectifiers having its input circuit damped, said one rectifier being of the delayed type.

3. In a modulated carrier energy receiver of the type including at least a carrier amplifier, a demulator and a modulation voltage amplifier all arranged in cascade; the improvement which includes a second demodulator arranged in cascade ALFONS ANDREATTA. 

